1. Field of the Invention
The present invention relates to a semiconductor device having a circuit including a thin film transistor (hereinafter referred to as a TFT), and to a method for manufacturing the same. For example, the invention relates to an electronic device on which an electro-optical device typified by a liquid crystal display panel or a light emitting display device having an organic light emitting element is mounted as its component.
In this specification, the term “semiconductor device” refers to a device in general that utilizes semiconductor characteristics to function, and electro-optical devices, semiconductor circuits and electronic devices are all included in the semiconductor device.
2. Description of the Related Art
In recent years, a technique for forming a thin film transistor (a TFT) by using a semiconductor thin film (thickness is approximately from several nm to several hundreds nm) formed over a substrate having an insulating surface has drawn attention. The thin film transistor has gained a wide application in electronic devices such as an IC and an electro-optical device, and particularly, development of a TFT as a switching element for an image display device has been hurriedly carried out.
As for applications utilizing such an image display device, a variety of applications is expected. The utilization for a portable device has drawn attention, in particular. Although a glass substrate and a quartz substrate are widely utilized at present, there is a disadvantage of being easily cracked and heavy. Moreover, a glass substrate and a quartz substrate are difficult to be made larger on the basis of mass-production, and these are not suitable for that. Therefore, forming a TFT element over a substrate having flexibility, typically, over a flexible plastic film has been attempted.
However, since the heat resistance of a plastic film is low, it cannot help lowering the highest temperature of a process. As a result, a TFT which has not so excellent electric characteristics compared with those formed over a glass substrates is formed at present. Therefore, a liquid crystal display device and a light emitting element having a high performance using a plastic film have not been realized yet.
Moreover, a method for peeling a peeling layer existing over a substrate with a separation layer therebetween from the substrate has been already proposed. For example, techniques described in Japanese Patent Laid-Open No. Hei 10-125929 and Japanese Patent Laid-Open No. Hei 10-125931 are techniques that a separation layer including an amorphous silicon (or a polysilicon) is provided, laser light is radiated by being transmitted through the substrate, and makes hydrogen contained in the amorphous silicon released, thereby generating a space-gap to separate the substrate. In addition, there is also a description in Japanese Patent Laid-Open No. Hei 10-125930 that by utilizing this technique, a liquid crystal display device is completed by pasting a peeling layer (in the gazette, referred to as a transferring layer) to a plastic film.
However, in the above-mentioned method, it is essential to use a substrate having high light-transmitting properties, and for transmitting light through the substrate and further, for the purpose of giving sufficient energy for releasing hydrogen contained in the amorphous silicon, the irradiation of laser light having comparatively large energy to the entire surface is necessary. Consequently, there is a problem that the peeling layer is damaged. Moreover, in the above-mentioned method, in the case when an element is manufactured over a separation layer, if heat treatment at a high temperature or the like is performed in the process for manufacturing the element, hydrogen contained in the separation layer is dispersed and reduced. In that case, even if the laser light is radiated on the separation layer, there is a possibility that the peeling is not sufficiently performed. Therefore, in order to maintain the amount of hydrogen contained in the separation layer, a problem that the processes after the separation layer formation are limited occurs. Moreover, in the above-mentioned gazette, there is also a description that, a light-shielding layer or a reflection layer is provided in order to prevent a damage to the peeling layer. However, in this case, it is difficult to manufacture a transmissive type liquid crystal display device. In addition, by the above-mentioned method, it is difficult to peel a peeling layer having a large area.
Accordingly, the applicant proposes a peeling technique and a transferring technique described in Japanese Patent Laid-Open No. 2003-174153.